Probably so that it's harder to flood the market with certain items. Lets say someone plays for a while and then caps poetics at 10k, they could spend all 10k on a material and then sell it all at once. Of course someone who is paying attention could stock up and do the same thing, but it at least adds steps to reach that point. Along the same lines, for those who aren't actively pre-buying materials for the Anima (and zodiac to a lesser degree) weapon, it slows down how much progress they can immediately make if they are capped and decide they wanna go for it. With 10k poetics, you are already over halfway there, with 2k, you still got work ahead of you.

And with yellow scrips, it means you can't immediately just buy nearly every craft book as soon as they drop, you'll only be able to afford one at a time before you have to spend more time grinding.

You might notice that these are measures to make you spend more time grinding, which is what the devs want since they want you to spend more time playing the game, and there can only be so much meaningful content. There might even be other reasons for the caps.